As is known, greater or lesser speeds of a ship are obtained by increasing or decreasing the rotative speed of the engine, that is the number of revolutions at which the propeller turns per unit of time.
It will be understood that, due to the fact that the driving torque is reduced as the rotative speed of the engine is reduced, certain rotative speeds cannot be utilized.
The design of the propeller of a ship is directly related to the engine, depending on the power and rotative speed thereof.
When the ship is to convey a particular cargo at a particular speed, the propeller and the engine are designed according to these needs. However, when the ship must use the complete power of its engine at speeds differing from the predetermined speed, a suitable response is not obtained due to the lack of adaptability of the propeller in view of this contingency.
The existence of variable-pitch propellers has put an end to such problem. In short, a variable-pitch propeller consists of a propeller having blades the pitch of which can be oriented without having to stop the ship.
In view of this innovation, an optimum driving torque is obtained at different engine speeds.
One of the most suitable applications of the variable-pitch propeller is its use in hauler fishing vessels which, due to their characteristics, have a sailing speed and another speed, which could be referred to as a working speed, for hauling the net during fishing.
In the first case, the ship should sail with an optimum driving torque which permits a rapid and economical speed.
In the second case, the speed is slower but the power needed is higher, due to the additional load resulting from the weight of fish being hauled or carried.
The variation in the draft of the blades of the propeller is the best solution and as such it was put into practice immediately by shipbuilders, so that newly constructed ships incorporate variable-pitch propellers.
The use of such propellers calls for the ship to already be designed therefore, since it is necessary to adapt the transmission shafts so that the necessary mechanisms which act on the blades to orient their draft are arranged therein.
Ships which have already been constructed must be modified so that variable-pitch propellers can be adapted thereto. However, the necessary modifying operations to be carried out affect the internal structure of the ship, they require a prolonged time in the shipyard, and the cost is high. For these reasons shipbuilders are unwilling to make such adaptations.
It is, therefore, desirable to find a solution which permits already existing ships incorporating a propeller having fixed blades to be transformed into ships incorporating a propeller having variable-pitch blades. However, this transformation should not affect the structure of the ship too greatly, to thereby reduce the expenses resulting from the inactivity of the ship and from the adaptation.